iio:adc:ti-ads124s08: Fix alignment and data leak issues.

One of a class of bugs pointed out by Lars in a recent review.
iio_push_to_buffers_with_timestamp() assumes the buffer used is aligned
to the size of the timestamp (8 bytes).  This is not guaranteed in
this driver which uses an array of smaller elements on the stack.
As Lars also noted this anti pattern can involve a leak of data to
userspace and that indeed can happen here.  We close both issues by
moving to a suitable structure in the iio_priv() data with alignment
explicitly requested.  This data is allocated with kzalloc() so no
data can leak apart from previous readings.

In this driver the timestamp can end up in various different locations
depending on what other channels are enabled.  As a result, we don't
use a structure to specify it's position as that would be misleading.

Fixes: e717f8c6 ("iio: adc: Add the TI ads124s08 ADC code")
Reported-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: Dan Murphy <dmurphy@ti.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200920112742.170751-9-jic23@kernel.org